industrial utilization of plastics, particularly the so-called engineering thermoplastics, has greatly increased over the past several decades. These materials have found their way into such diverse applications as home and office furniture, airplane interiors, cabinetry and casing for electronic and computer systems and various components for automobiles, machines and cookware, inter alia. Some of these plastics, however, are not tough enough for the demanding applications which continue to spring forth from the fertile minds of cost-conscious design engineers. In this regard, considerable effort has been devoted toward the improvement of mechanical properties of the plastics, the enhancement of impact strength being particularly prized. To a limited extent, these difficulties have been overcome by the addition of various rubber compositions to thermoplastic resins.
For example, Liang et al., in U.S. Pat. No. 4,888,390, issued Dec. 19, 1989, showed that certain rubbers could be used to improve the crack and/or impact resistance of a poly(phenylene sulfide) (PPS) resin. In a similar approach, U.S. Pat. No. 3,920,770 to Nakashio et al., issued on Nov. 18, 1975, teaches a poly(phenylene ether) (PPE) resin modified with various rubbery polymers. These compositions have improved elongation and impact resistance relative to unmodified resin.
Designers employing these plastic materials also place a high premium on fire retardancy since accidental fires continue to extract a heavy toll on life and property. Here, the thermoplastic and thermosetting resins are less than satisfactory due to their organic (i.e., inherently combustible) nature. This deficit has also been addressed, most notably by incorporating various halogen or phosphorous fire retardant compounds in the plastic composition. A hydrated metallic compound, such as a hydrated alumina filler can also be used as fire retardant component, either by itself or in combination with the aforesaid compounds. Unfortunately, such tactics present disadvantages of their own: the addition of hydrated fillers can detract from the mechanical properties of the modified plastic while many of the halogen and phosphorous compounds are undesirable from a toxicological perspective. Additionally, even though the halogen compounds do impart flame resistance, their products of combustion are notoriously corrosive. Therefore, sensitive electronic components which have been exposed to the fumes of burned plastics containing such compounds can suffer extensive corrosion damage even though they are otherwise unaffected by the heat of the fire. The deleterious effects can occur months after the incidence of fire and the use of these compounds could foster a false sense of security. There is therefore a need for modified plastic systems which place less reliance on these conventional means of achieving fire retardant properties. Some progress toward this end has been made by modifying plastics with certain silicone components.
Thus, for example, a flame retardant PPE resin composition containing an aromatic alkenyl resin, a polyorganosiloxane graft copolymer, a phosphate and a particulate silicic acid is disclosed in U.S. Pat. No. 5,064,887 to Yamamoto et al., issued on Nov. 12, 1991.
In U.S. Pat. No. 4,387,176, issued on Jun. 7, 1983 to Frye, a thermoplastic flame retardant composition is disclosed which employs a combination of a silicone fluid or gum, a metal organic compound and a silicone resin as the modifier for the thermoplastic. A similar system is shown by Frye et al. in U.S. Pat. No. 4,536,529, issued on Aug. 20, 1985. In the latter patent, the modifying components include a silicone fluid, a metal soap precursor and a silicone resin. These compositions are said to offer simpler processing and improved impact resistance over unmodified thermoplastics.
Smith et al., in U.S. Pat. No. 5,017,637, issued on May 21, 1991, teach a fire-retardant thermoplastic composition comprising an olefinic copolymer or terpolymer, a polyorganosiloxane, a metal oxide hydrate and a dialdehyde. This composition finds utility in molding and extrusion applications.
A flame retardant polymer composition which is essentially free of halogen compounds and organometallic salts is disclosed in European Patent Application 0393959 to BP Chemicals Ltd., published on Oct. 24, 1990. This contribution to the art employs a combination of a silicone fluid or gum and an inorganic filler selected from compounds of a Group IIA metal, to modify certain copolymers of ethylene. The filler is typically a compound such as magnesium oxide, magnesium carbonate or calcium carbonate and the modified polymers are useful in wire and cable applications.
In the elastomer art, it is also known to prepare organosiloxane compositions in the form of a free-flowing powder prepared from a high consistency "gum-type" polydiorganosiloxane and a reinforcing filler. There is, however, no suggestion to combine these with a thermoplastic resin as disclosed herein.
In accordance with the teaching of Link and Scarbel in U.S. Pat. No. 3,824,208, issued Jul. 16, 1974, a powdered material is obtained by first reducing the particle size of the polydiorganosiloxane and then mixing the particles with at least 15 parts by weight of a reinforcing filler at a temperature of from 0.degree. to 100.degree. C. and under particular shear conditions.
Japanese Patent Publication No. 2/102007 to Toshiba Silicone Co., published on Apr. 13, 1990, teaches pelletizing a high consistency or "gel" type vinyl-containing polydiorganosiloxane and then blending the resultant pellets with a filler. A processing aid is included to prevent a phenomenon referred to as "creping" or "crepe hardening." The resultant composition is then mixed using a high speed rotating blade at 10.degree. to 100.degree. C. to produce a free-flowing powder.
Elastomers prepared from silicone rubber powders according to the above cited teachings of Link and Scarbel and Japanese Patent Publication No. 2/102007 were found to have a number of shortcomings, such as the presence of undesirable gel particles which are discernable to the unaided eye as clear spots when the powdered rubber is combined with a suitable dye and massed into a thin section. This gel problem was essentially overcome by the discoveries of Bilgrien et al., as disclosed in a application for patent entitled "Storage Stable Organosiloxane Composition and Method for Preparing Same," Ser. No. 790,043, filed on Nov. 12, 1991, now U.S. Pat. No. 5,153,238, assigned to the assignee of the present invention and hereby incorporated by reference. The silicone rubber powder compositions of Bilgrien et al. have an average particle size of 1 to 1000 microns and are prepared by blending a high consistency polydiorganosiloxane into a quantity of fluidized reinforcing filler that is heated to a temperature of &gt;100.degree. C. to 200.degree. C. prior to, or immediately following, introduction of the polydiorganosiloxane. The resultant rubber powders additionally exhibit excellent storage stability and can be subsequently massed and cured to yield substantially gel-free elastomers having excellent physical properties.
The particular silicone rubber powder prepared according to the disclosure of Bilgrien et al., cited supra, was found to be useful as a modifier for PPE resins and provided unexpected improvements in impact resistance and processability for only these polymers. This discovery was disclosed by Romenesko et al. in copending application for patent entitled "Poly(phenylene ether) Resin Modified with Silicone Rubber Powder," Ser. No. 793,877, filed on Nov. 18, 1991 and assigned to the assignee of the present invention.
All of the above mentioned improvements in the modification of plastic resins notwithstanding, there is still a need for plastic materials having a greater degree of fire retardancy. Moreover, recent trends in the art suggest that widely-accepted test methods used to evaluate the fire retardant character of plastics are not predictive of their real life fire hazard. Conventional tests, such as Limited Oxygen index (LOI), which is a measure of the minimum oxygen content of the atmosphere capable of sustaining combustion of the sample, and Underwriters Laboratory method UL-94, wherein certain burn properties of a vertical or horizontal test piece are determined-(described in greater detail infra), only provide gross measures of flame resistance. The latter test is the industry's method of choice and various modifying agents discussed above are typically added to plastics in order to pass this test.
However, neither of the above methods offers specific information about the rate of heat generation in a fire; nor do these tests provide information about the rate of smoke generation or the production of toxic gases as the samples are burned. It has been well documented that these are the predominant elements responsible for death and injury in a real fire situation. Thus, although the conventional tests may be well established and easy to carry out, they are not good indicators of the actual liability associated with the burning of a given material since they do not measure the above mentioned elements of the combustion process.
A more promising evaluation of the critical parameters (rate of combustion and the evolution of smoke and carbon monoxide) has recently been developed. This method, which has been codified as American Society for Testing and Materials standard ASTM E 1354-90, plays a key role in the instant invention and is described in greater detail below. It employs a so-called cone calorimeter to obtain a quantitative display of the above mentioned combustion elements as a function of burn time. Using this method, the skilled artisan can quickly predict the relative combustion hazard of a given new plastic formulation.